Connected: An Internet Encyclopedia
RFC 1123 - 5.3.8 Maximum Message Size

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### 5.3.8 Maximum Message Size

Mailer software MUST be able to send and receive messages of at least 64K bytes in length (including header), and a much larger maximum size is highly desirable.

DISCUSSION:

  Although SMTP does not define the maximum size of a message, many systems impose implementation limits.

  The current de facto minimum limit in the Internet is 64K bytes. However, electronic mail is used for a variety of purposes that create much larger messages. For example, mail is often used instead of FTP for transmitting ASCII files, and in particular to transmit entire documents. As a result, messages can be 1 megabyte or even larger. We note that the present document together with its lower- layer companion contains 0.5 megabytes.

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Connected: An Internet Encyclopedia
RFC 1123 - 5.3.8 Maximum Message Size